I don’t think it’s entirely clear. But a likely reason is that when your liver is processing alcohol, it breaks it down into a chemical called acetaldehyde. This chemical is what we call a carcinogen (meaning that it can promote cancer development), because it increases the rate at which mutations happen, so it increases the chances of you getting dangerous mutations that lead to cells growing out of control and becoming cancerous.
In fact the World Health Organisation classifies alcohol as a carcinogen – a cancer causing substance. When alcohol is in the body, it can create chemicals which damage the DNA in your cells, causing that cell to mutate. It is this mutation which creates the potential for the cell to turn cancerous.
You should, for a number of reasons, limit how much alcohol you drink, and ensure that you drink in moderation. You should also avoid smoking while you drink, because this increases the risk of developing mouth cancer: